1. Field of Invention
The technology of Organic Light Emitting Diode (OLED) consists of three families namely, (i) Fluorescent family (ii) Phosphorescent family and (iii) Polymer family. Each family has its own merits and demerits. Fluorescent family, based on small molecule, has a long life time and hence 90% of the commercial product of OLED technology on the market is based on fluorescent family. Phosphorescent family, based on the dopants of heavy metal-atom-complex such as iridium complex, has the best efficacy but does not possess long life, especially the blue color OLED. Polymer family, based on long molecule, has the potential for low cost in manufacturing but does not have long life. Recently the focus is more on fluorescent and phosphorescent families. By combining the two technologies of both families, it is possible to derive the benefit of both families namely, good efficacy and long life.
For flat panel display application of OLED, there are two methods that are employed. One method is to incorporate discrete red pixel, discrete blue pixel and discrete green pixel of OLED and drive these pixels to generate full color video images. The other method is to incorporate white OLED pixels, consisting of the mix of red-blue-green OLED, and employ red, blue and green color filters over the white OLED layer. By selectively energizing these pixels, full color video images can be displayed.
Another important field of application for OLED is General lighting. High luminance of around 20,000 cd/m2 is required for application in lighting. To obtain this high luminance, the current density in OLED can reach 30-40 mA/cm2. At this current density, OLED should possess long life. But it is not the case. OLED is known to exhibit short life at high current density and long life at low current densities. Operating OLEDs at low current density and still obtaining high luminance is possible by connecting several OLEDs in series. A monolithic series OLED is also a routine process being done in the name of ‘tandem OLED’. Applications like lighting and displays demand long life, high efficacy and high brightness, coupled with low price, from OLEDs. To meet this demand, a combination of the advantages of all the three families of OLED can be exploited.
2. Description of Prior Art
The life of white OLED from fluorescent family and phosphorescent family is given in Table I and Table II. Table I shows that OLED, from phosphorescent family that emits blue light, has a life time of 3000 hours when life-tested with a starting luminance of 500 cd/m2. The OLED from fluorescent family that emits blue light, has a life time of 12000 hours when life tested with a starting luminance of 1000 cd/m2. It is well known that the luminance increases with current density. Thus it is clear that the blue light emitting OLED from phosphorescent family will have poor life if the starting luminance is increased to 1000 cd/m2 instead of 500 cd/m2. The efficacy of phosphorescent OLED that emits blue light is higher than that of the blue light emitting OLED from the fluorescent family. Table II gives the life time of white OLED from phosphorescent family and fluorescent family. The data in the first two rows represent the white light emitting OLED that contains red, blue and green light emitting layers. The data in the last row represents the white light emitting OLED that contains both the families of OLED in that blue light emitting layer is made out of fluorescent family and red+green (yellow) light emitting OLED is made out of phosphorescent family. It is clear that pure phosphorescent white OLED has high efficacy of 64 lm/w, but has a life of only 10,000 hours when the starting luminance for life test is set at 1000 cd/m2. The pure fluorescent white OLED on the other hand has a long life of 50,000 hours but the efficacy is only 19.5 lm/w. The hybrid white OLED that contains the fluorescent family and the phosphorescent family has life time and efficacy in between that of pure fluorescent and pure phosphorescent family.
TABLE IBlue OLEDLifeInitial(50% of initialOLED FamilyBlue (x, y)LuminanceEfficacyluminance)Phosphorescentx = 0.16; 500 cd/m2 11 cd/A 3000 Hrs(Ref: 1)y = 0.29Fluorescentx = 0.14; y =1000 cd/m27.2 cd/A12000 Hrs(Ref: 6)0.16
TABLE IIWhite OLEDEstimatedfunctional LifeWhiteInitialEfficacy(50% of initialOLED Family(x, y)Luminancelm/wluminance)Phosphorescentx = 0.371000 cd/m26410,000 Hrs(Ref: 5)y = 0.42Fluorescentx = 0.28;1000 cd/m219.550,000 Hrs(Ref: 9)y = 0.33Phosphorescent-x = 0.3681000 cd/m23031,000 HrsFluorescenty = 0.385Hybrid (Ref: 8)
The life test procedure for OLED is typically as follows:    (i) Set the initial luminance of 1000 cd/m2 and in DC drive condition operate the OLED continuously.    (ii) Monitor the luminance after 24 hours and thereafter at interval of 100 hours.    (iii) Continue life test for 1000 hours and plot the luminance vs time for 1000 hours.    (iv) From the trend line, extrapolate the curve to reach 50% of the initial luminance of 1000 cd/m2.    (v) Estimate the life based on 50% level.    (vi) Since the test is continuous and the real functional life is not continuous, use an empirical power law of 1.5, for the life estimated for 50% level, and arrive at the final estimated life.
The number of hours reported in Table I and II is based on the above procedure.
For obtaining white light from OLED, the practice in the industry is to employ hybrid white OLED (HW-OLED) that contains fluorescent family for blue light emission and phosphorescent family for yellow (red+green) light emission. This yields long life typical of fluorescent family and high efficacy typical of phosphorescent family. This is the Prior art. For applications in illumination and application in TV, the luminance requirements are higher than reported in Table I and Table II. For example a compact fluorescent lamp yields a luminance of 20,000 to 30,000 cd/m2. For TV application the final screen should yield a luminance of 500 cd/m2 but to obtain this luminance, with anti-reflection coating and color filters intervening the viewer and OLED, the OLED need to yield a luminance >2000 cd/m2. To obtain higher luminance from the structure of HW-OLED the current density needs to be increased. The ratio of green:red:blue in white light is 64%:28%:8%. The luminous flux in all these three colors needs to increase in the same ratio. The current density required to increase the flux in blue light emission from fluorescent family is in the range of 10 mA/cm2 to 20 mA/cm2. This current density is too high for the materials emitting yellow (red+green) light from the phosphorescent family and the life of the phosphorescent materials will deteriorate resulting in poor life of HW-OLED. Materials from the phosphorescent family exhibit satisfactory life when operated below a current density of 7 mA/cm2. If the current density is increased to the same level as the blue light emitting fluorescent layer, the increase in the luminous flux from yellow light (red+green) will far exceed the ratio specified for white and thus the chromaticity will go out of control, in addition to shortening of life. Even at low current density (<7 mA/cm2) of operation, doping control during the processing needs to be accurately executed to maintain the chromaticity. These are the drawbacks of the prior art. The industry is not able to find any solution for this problem to-date.